The present invention relates generally to adsorption dryers using a desiccant to dehydrate gases. Specifically this invention relates to a desiccant gas dryer of the kind having a pair of sorbent or desiccant beds, wherein one bed adsorbs water vapor as the other bed is regenerated. More particularly, the invention relates to the regeneration of the desiccant using a combination of the heat of compression and a dry gas purge sweep.
It is frequently desirable and often necessary to remove at least the greater portion of water and some times the last traces of moisture from gases. For example, in natural gas pipeline transmission systems, hydrate formation can be eliminated by removing a sufficient amount of the water vapor so that the dew point of the gas is at least as low as the minimum temperature to which the gas will be exposed at the maximum pressure utilized in the system. Also, many processes wherein gases such as air or low molecular weight hydrocarbons are used as reagents require that said gases be essentially anhydrous. Even in situations where the gas is not utilized as a reagent, such as in pneumatic control systems where so-called "instrument air" is employed as the power fluid, it is required that the air be essentially anhydrous in order to avoid condensation and ice formation during winter conditions.
Ambient air even in the driest areas contains moisture. Relative humidity is the term used to indicate the amount of water vapor present in a volume of air at a given temperature compared to the amount of water vapor that air is capable of holding at that temperature. For example, if the vapor content is 70% of the moisture holding capacity of the air at a certain temperature, then the relative humidity would be 70%.
When the temperature of saturated air is lowered, some of the vapor condenses or changes to liquid. As a rule of thumb, the water holding capacity of a gas doubles with every 20.degree. F. rise in temperature. Thus the higher the temperature, the more vapor that can be held by a given volume of air. Dewpoint is the air temperature, at any given pressure, at which water vapor begins to condense into a liquid. The lower the dewpoint, the less water there is in the air. The water holding capacity of a gas is also dependent upon volume. If the actual volume of a saturated gas is increased by reducing the pressure, then the water holding capacity of that gas increases.
As previously mentioned, the presence of water in a pneumatic system can be very detrimental, the water becomes the agent for a contamination chain. Rust and scale rapidly degrades the efficiency of the system by clogging the orifices and jets of pneumatic equipment; rust collects in bends and pockets creating excessive pressure drops; pipe connectors are weakened and leaks are encouraged; rust can break loose, pass down stream and render pneumatic instruments inoperative.
Desiccant dryers are commonly employed to dry compressed air used for pneumatic controls in manufacturing plants. These dryers typically have a pair of desiccant chambers each containing, for example, silica gel, alumina or zeolitic molecular sieves adsorbents. In such systems, the beds alternately dry the process stream and then are regenerated.
Once a desiccant becomes moisture laden, additional moisture cannot be removed from the gas during the adsorption cycle until the desiccant has been regenerated. In general, the more thorough the regeneration, the better the quality of the effluent air, in terms of having a low dewpoint, and the more moisture which can be adsorbed before the next regeneration. It is generally understood that the quality of regeneration is dependent upon the temperature and dryness of the regenerating media. See Arnold L. Weiner, "Drying of Liquids and Gases," page 6, Chemical Engineers, Sept. 16, 1974. It is known that adsorbents may be regenerated by a variety of techniques, including heating the adsorbents or desiccants, passing a dry purge gas through the desiccant or by passing heated gas through the bed. Such techniques may be referred to as heat-regenerated, pressure swing and heat-of-compression systems, respectively, with the latter designation deriving from a known practice of using the heat generated by compressing the gas to be dried.
One basis for comparison of the various regeneration techniques is the amount of energy required for regeneration. Very generally, pressure swing regeneration typically employs the most energy (through use of compressed, product air as the regeneration medium), but offers very good dewpoint performance.
Heat regenerated dryers typically employ somewhat less regeneration energy, with some sacrifice in dewpoint performance. Heat of compression dryers are susceptible to wide swings in regeneration costs depending on the condition (moisture content and temperature) of the compressor inlet air. "The Truth About `Waste Heat` Air Dryers," page 105, Machine Design, Mar. 27, 1987, incorporated herein by reference. In dry winter months, heat of compression dryers can give low dewpoints more economically than either heat-regenerated or pressure swing systems. In humid summer months, however, conventional heat of compression dryers may require more energy than either heat regenerated or pressure swing dryers for comparable dewpoint performance. This is explained in more detail below.
A prior art practice of heat of compression desiccant dryers as disclosed, for example, in U.S. Pat. No. 3,205,638 is to utilize the compressor discharge gas heat to fully achieve regeneration. Specifically, in prior art heat of compression dryers, the compressor discharge is typically passed through a regenerating desiccant chamber where it heats and desorbs moisture from the desiccant requiring regeneration. The discharge gas from the regenerating chamber passes through a cooler where the gas is cooled and the water vapor is condensed out. Following the cooler is a separator which separates and collects the condensed water which is discharged or vented from the separator valve. The gas then passes through the active or drying chamber where water vapor is further removed to provide for a dry gas delivery at the dryer outlet. At conditions of high humidity, as in summer, however, the desiccant cannot be satisfactorily regenerated to a level which will produce minus 40.degree. F. dewpoints when drying. This has lead to the use of a booster heater to raise the temperature of the compressor discharge to a level that will result in the regeneration characteristic of providing the minus 40 .degree. F. dewpoint desired. The use of such a booster heater, of course, increases the regeneration energy. A comparison of regeneration energy requirement for various types of dryers is given in Table I. The table shows the regeneration energy requirements for a 2000 scfm. 100 psig air dryer dryer employing silica gel desiccant and utilizing a 400 h.p. 2-stage oil-free compressor, operating with an ambient temperature of 100.degree. F. and 50% relative humidity with a design effluent dewpoint of -40.degree. F. Along with the heat of compression dryers are listed the energy requirements for an electrically heated air dryer and a heaterless regenerated dryer.
TABLE I ______________________________________ Heater Energy Dryer Type Size Requirement Remarks ______________________________________ HOC 93KW 93KW 450.degree. temperature required 1 Hour Cycle for regeneration air quality, outlet temperature and dewpoint spikes at chamber switching HOC 23KW 23KW 450.degree. temperature required 8 Hour Cycle for regeneration air quality, outlet temperature and dewpoint spikes at chamber switching Electric 38KW 38KW Heater and side stream 8 Hour Cycle flowrate must be matched Side-Stream to produce regeneration Regeneration heat quantity and temper- ature humidity quality, temperature and dewpoint spikes at chamber switching Heaterless NONE 45KW 300 SCFM purge air (PSA) Dry consumption requires Purge Air 45KW compressor energy. Compressor Dry air delivery is 1700 Energy SCFM. No temperature spikes, no dewpoint spikes. ______________________________________
As explained above, the requirements for heat-of-compression regeneration are subject to wide variation depending upon the temperature and humidity of the ambient air feed to the compressor. By way of example, with the ambient air feed at 60.degree. F. and 50% relative humidity (as opposed to the 100.degree. F. and 50% relative humidity conditions for Table I), the booster heater requirement for the one hour heat of compression example above would be 31KW. This requirement results from a 300.degree. F. regeneration temperature necessary to achieve a -40.degree. F. effluent dewpoint.